The fabrication of high aspect ratio features in silicon is used extensively in the manufacture of micro-electro-mechanical (MEMS) devices. Such features frequently have depths ranging from tens to hundreds of micrometers. To ensure manufacturability, the etching processes must operate at high etch rates to maintain reasonable throughputs, along with other performance requirements such as smooth etch profiles.
Conventional, single step, plasma etch processes cannot simultaneously meet these needs, and time division multiplex etch processes have been developed. Time division multiplexed (TDM) approaches for etching silicon have been described by Suzuki et al. (U.S. Pat. No. 4,579,623), Kawasaki et al. (U.S. Pat. No. 4,795,529) and Laermer et al. (U.S. Pat. No. 5,501,893). TDM etch processes typically employ alternating etching and deposition steps. For example, in etching a silicon (Si) substrate, sulfur hexafluoride (SF6) is used as the etch gas and octofluorocyclobutane (C4F8) as the deposition gas. In an etch step, SF6 facilitates spontaneous and isotropic etching of silicon (Si); in a deposition step, C4F8 facilitates protective polymer passivation onto the sidewalls as well as the bottom of the etched structures. In the subsequent etching step, upon energetic and directional ion bombardment, the polymer film coated in the bottom of etched structures from the preceding deposition step will be removed to expose the silicon surface for further etching. The polymer film on the sidewall will remain, inhibiting lateral etching. TDM processes cyclically alternate between etch and deposition process steps to enable high aspect ratio structures to be defined into a masked silicon substrate at high etch rates.
In each process step, gases (for example, SF6 and C4F8) are introduced into the reaction chamber through a gas inlet at flow rates specified in the process recipe. TDM etch process are typically performed in high density plasma reactors (e.g., inductively coupled plasma (ICP), electron cyclotron resonance (ECR), etc.).
TDM process recipes consist of a series of process loop(s) and steps. Each loop consists of two or more process steps controlling the process variables (e.g., gas flow rates, chamber pressure, RF powers, process step times, chamber temperature, wafer temperature, etc.). The steps within a loop are repeated a number of times before executing the next step or loop in the overall process recipe. It is known to make changes to the process step parameters as a loop repeats to improve etch performance, this is known in the art as process morphing (see Teixeira et al. U.S. Pat. No. 6,417,013).
Pressure control is an important component of etching and deposition processes. The flow rate and pressure of the process gases present in the chamber must be carefully controlled to provide the desired deposition and etch characteristics for a repeatable manufacturing process.
A TDM plasma reactor evacuation system typically comprises a turbo pump separated from the reaction chamber by a throttle valve. A pressure controller uses reactor chamber pressure data from a manometer to control a throttle valve. The controller opens or closes the throttle valve to increase or decrease the vacuum supplied from the turbo pump to the reaction chamber. In this manner, the controller maintains the desired pressure in the reaction chamber. During the TDM process chamber pressure set points and gas flow rates cyclically alternate within the process loops. The gas flows can be either single component or mixtures of multiple components. The pressure controller must adjust the throttle valve position to compensate for these changing flow and pressure conditions. Ideally, the pressure controller adjusts the throttle valve position to instantly achieve the pressure set point without pressure set point overshoot or undershoot.
Throttle valves and controllers, currently available, typically operate in either Pressure Control mode or Position Control mode. In the Pressure Control mode the controller monitors the pressure in the reaction chamber and maintains the set point pressure by adjusting the position of the throttle valve (i.e., closed loop pressure control). In position control mode the controller positions the throttle valve to a set point position without monitoring the chamber pressure (i.e., open loop pressure control).
A number of groups have looked at means for process control in plasma chambers. Kessel et al. (U.S. Pat. No. 4,253,480) describes a pressure regulator that employs an adjustable solenoid valve to control pressure. Kessel teaches the fundamental mechanism dictating the operations of many throttle valves used in vacuum chambers. The actual pressure in a container is measured and converted to electrical signals. A comparator generates a regulation signal that represents the difference between the actual pressure and a command pressure. A regulator uses the regulation signal to direct the valve in such a manner that the valve member is adjustable between intermediate positions within a range between the open and closed positions of the valve. In fact, the throttle valves used in TDM process tools are operated following such principles. However, as described earlier, the inability to control pressure during the transition of the constantly alternating TDM process steps is the real issue, and cannot be addressed by Kessel's technique.
Kaveh et al. (U.S. Pat. No. 5,758,680) and McMillin et al. (U.S. Pat. No. 6,142,163) describe the use of a ballast port for inserting gas into the evacuation system to compensate the pressure fluctuations in the reaction chamber so as to minimize throttle valve movement between different process steps. They further disclose a method to reduce the time for gas stabilization in a vacuum chamber. A throttle valve is first pre-positioned to the desired position. The desired position is estimated using pre-determined estimation curves. Then for a specified period of time, proportional and derivative (PD) control is enabled to control throttle valve movement. Then proportional, integral and derivative (PID) control is enabled to regulate throttle valve movement. The examples taught in the disclosure show that the time period for pressure stabilization is reduced from ˜20 seconds to at least 3–5 seconds. While Kaveh and McMillin contemplate the change of gas flow rates and pressures when process steps change from one to the next, the use in cyclical and alternating TDM processes is not taught. Additionally, many TDM processes employ alternating process steps which last only a few seconds or shorter, which makes pressure control impractical using the disclosed technique.
Brown et al. (U.S. Pat. No. 6,119,710) describes the use of adjustable gas flow into the reaction chamber to compensate the pressure variations in the chamber. However, in many TDM processes, changing process gas flow rate during a process step is undesirable.
Beyer et al. (U.S. Pat. No. 5,944,049) describes regulating chamber pressure by controlling either the exhaust pressure at the exhaust side of a vacuum pump or the internal pressure at a compression stage of the first vacuum pump. Adjustments on vacuum pumping speed or injection of inert gas into the exhaust side or the compression stage of a vacuum pump are used to control reaction chamber pressure. Beyer does not teach how to use this technique in TDM processes.
Puech (U.S. patent application Ser. No. 20020168467) describes a way to control pressure by injecting passive control gas at a complementary flow rate into an area near the evacuation port. The flow rate of the controlled passive gas is regulated so as to maintain the total gas flow into the vacuum enclosure at a substantially constant rate. While Puech teaches the control of pressure in the TDM processes that employ process steps on the order of one second, the method does not teach the use of actively regulating throttle valve in pressure control.
The current methods of pressure control for TDM processes, Pressure Control and Position Control, have limitations. One problem with pressure control mode in a TDM process is that, in practice, there is typically a trade off between achieving fast pressure response time while minimizing set point deviations. Fast response times are possible at the expense of a period of pressure set point overshoot. Optimizing available Pressure Control mode algorithms to minimize set point overshoot results in slow response times. As the TDM step durations decrease, the time spent trying to reach the recipe specified set point becomes a significant fraction of the processing time.
A problem with the current method of Position Control mode in a TDM process is unacceptably long pressure response times. While position mode minimizes process overshoot, the slower response times result in the chamber pressure spending a large fraction of the process time approaching the requested set point value (i.e., out of compliance with the recipe specified set point).
Another problem with the position control mode method is that it is an open loop pressure control algorithm. Therefore, there is not any correction for perturbations in gas flow or pumping efficiency. These perturbations tend to cause the process pressure, and subsequent process performance, to vary with time.
Therefore, there is a need for a pressure control means for TDM processes, preferably for those processes that employ process steps that are a few seconds or less in duration.
Nothing in the prior art provides the benefits attendant with the present invention.
Therefore, it is an object of the present invention to provide an improvement which overcomes the inadequacies of the prior art devices and which is a significant contribution to the advancement of the semiconductor processing art.
Another object of the present invention is to provide a method for anisotropically etching a feature in a substrate comprising the steps of: subjecting the substrate to an alternating cyclical process within a plasma chamber, said alternating cyclical process having an etching step and a deposition step; introducing into said plasma chamber a first process gas for depositing a film onto the substrate during the deposition step of said alternating cyclical process; introducing into said plasma chamber a second process gas for etching the substrate during the etching step of said alternating cyclical process; regulating pressure of said plasma chamber by setting a throttle valve at a predetermined position set point for a predetermined period of time during at least one step of said alternating cyclical process; igniting a plasma for a recipe period of time for the deposition step of said alternating cyclical process and the etching step of said alternating cyclical process; enabling a closed loop pressure control algorithm after said predetermined period of time expires; and controlling pressure at a recipe specified pressure set point in said plasma chamber through a closed loop pressure control for a period that lasts the remaining time of the step.
Yet another object of the present invention is to provide a method of pressure control in a time division multiplex process comprising the steps of: regulating a process pressure in a vacuum chamber in at least one step of the time division multiplex process by setting a throttle valve at a predetermined position set point for a predetermined period of time; introducing into said vacuum chamber at least one process gas; enabling a closed loop pressure control algorithm after said predetermined period of time expires; and controlling pressure at a recipe specified pressure set point through a closed loop pressure control for a period that lasts the remaining time of said step of the time division multiplex process.
Still yet another object of the present invention is to provide a method for controlling pressure in a vacuum chamber, the method comprising the steps of: regulating a process pressure in the vacuum chamber by setting a throttle valve at a predetermined position set point for a predetermined period of time; introducing into said vacuum chamber a gas; enabling a closed loop pressure control algorithm after said predetermined period of time expires; and controlling pressure at a recipe specified pressure set point in said vacuum chamber through a closed loop pressure control.
The foregoing has outlined some of the pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.